Controlled phase synthesis of VmOn in differing oxidation states using a simplified formic acid process, quantified with a new generalized index designed for use with public domain material process information

Abstract

An environmentally benign approach for the synthesis of vanadium pentoxide (V₂O₅), vanadium dioxide (VO₂) and divanadium trioxide (V₂O₃) sols, potentially suitable for device applications, is reported. Synthesis of vanadium oxides is challenging because of their tendency to exist in multiple (+2 to +5) and mixed oxidation states. However, a simple approach has been developed successfully using formic acid and citric acid, as a viable scalable and sustainable process. Crystalline oxides of vanadium were obtained after annealing of sols under controlled temperature and atmospheric conditions. Chemical composition of the synthesized V₂O₃ was inferred from Raman spectroscopy; VO₂ did not show any Raman peak at room temperature except a feature at 226 cm⁻¹, while no significant information was obtained from the Raman spectra of V₂O₃. When excited at 405 nm, V₂O₅ and VO₂ exhibit photoluminescence at 558 nm and 467 nm, respectively, while V₂O₃ shows radiative recombination at 446 and 473 nm. Particles of V_mO_n exhibit broad absorption bands as predicted by absorption spectroscopy. Sols of V₂O₅ and VO₂ were found to be stable for more than six months, while V₂O₃ shows instability in air. A free-standing V_mO_n solution was used as an active layer in metal insulator metal (MIM) structures to explore possible device applications. To evaluate the suitability of this process, a dimensionless composite index of greenness (G) of processes is proposed, based on commonly held metrics of the desirable characteristics of green processes. This index is amenable to derivation using restricted process information customarily available in the literature. The process is found to exhibit greenness indices of 0.744 and 0.585 for V₂O₅ and VO₂, respectively. Other published processes for the synthesis of these oxides are also quantified using our index.